1. Field of the Invention
The present invention relates to an optical pickup apparatus that performs operation of reading a signal recorded in an optical disc and operation of recording a signal in the optical disc by using laser light, and an objective lens used in the optical pickup apparatus.
2. Description of the Related Art
There are known an optical disc device capable of performing signal reproduction operation and signal recording operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer of an optical disc is known. As such an optical disc device, a device using an optical disc called CD or DVD is widely known in general, but that using an optical disc with improved recording density, that is, a Blu-ray standard disc has been developed in these days.
Infrared light with a wavelength of 780 nm is used as laser light for performing operation of reading a signal recorded in the CD-standard optical disc, while red light with a wavelength of 650 nm is used as laser light for performing operation of reading a signal recorded in a DVD-standard optical disc.
In contrast to such CD-standard and DVD-standard optical discs, laser light with a short wavelength, such as a blue-violet light with a wavelength of 405 nm, is used as laser light for performing operation of reading a signal recorded in the Blu-ray standard optical disc.
The thickness of the protection layer is 0.1 mm which is provided on an upper face of the signal recording layer in the Blu-ray standard optical disc, and the numerical aperture of the objective lens is set at 0.85 which is used for the operation of reading a signal from this signal recording layer.
In order to reproduce a signal recorded in the signal recording layer provided in the Blu-ray standard optical disc or to record a signal in the signal recording layer, a diameter of a laser spot formed by converging the laser light is required to be reduced. Since an objective lens used for obtaining a desired laser spot shape has not only a larger numerical aperture but also a shorter focal length, a radius of curvature of the objective lens is reduced.
The optical pickup apparatus includes a laser diode for emitting laser light with a wavelength corresponding to each of the above-mentioned standards and an objective lens for converging the laser light emitted from the laser diode to the signal recording layer provided in each of the optical discs. As a material of such an objective lens, glass is used in general, but synthetic resin has been often used in order to produce inexpensive lenses, recently.
Also, a configuration is made so as to be capable of performing a control operation for focusing the spot of the laser light emitted from the optical pickup apparatus onto the signal recording layer in the optical disc, i.e., a focusing control operation or a control operation for allowing the spot of the laser light to follow a signal track, i.e., a tracking control operation.
Also, in the optical pickup apparatus, spherical aberration is caused by the thickness of a protection layer located between a disc face, which is a laser light incident face of the optical disc, and the signal recording layer, thereby resulting in such a problem that the signal reproduction operation or signal recording operation cannot be performed normally. As a method for solving this problem, such a technique has been developed that the spherical aberration is corrected by moving a collimating lens provided between the laser diode and the objective lens in an optical-axis direction (See Japanese Patent Laid-Open Publication No. 2005-338684, Japanese Patent Laid-Open No. 2004-14042).
Though a price of the optical pickup apparatus can be reduced by manufacturing an objective lens by using synthetic resin, the synthetic resin has a characteristic that its properties, particularly transmission characteristics, are changed according to the wavelength of the laser light to be used. In the case where an objective lens for converging laser light conforming to the Blu-ray standard, i.e., blue-violet laser light with a wavelength of 405 nm, is manufactured by using such a synthetic resin material that is used for an objective lens for converging laser light conforming to the DVD standard, i.e., red laser light with a wavelength of 650 nm, for example, a synthetic resin material called APL5014DP by Mitsui Chemicals Inc., such a property is exhibited that transmittance of the objective lens is reduced according to elapsed time of irradiation of the blue-violet laser light.
Such transmittance is decreased as an output of the laser light becomes greater, and thus, this is a significant problem in an optical pickup apparatus in which an output of the laser light is increased such as an optical pickup apparatus that performs the recording operation.
If the transmittance of the objective lens is reduced, the laser light converged to the signal recording layer included in the optical disc is decreased in intensity, so that the operation of reproducing a signal recorded in the optical disc or the operation of recording a signal cannot be performed normally. In order to solve such problem, a synthetic resin material affected in a less extent by the blue-violet laser light such as a resin material called ZEONEX340R by Zeon Corporation is used as a material of the objective lens, for example, however, such a material is expensive, causing a problem that the optical pickup apparatus cannot be reduced in price.
Moreover, in order to increase the radius of curvature of the objective lens (that is, to flatten the curvature) and to decrease the thickness thereof, the objective lens is required to be configured by a resin material with a high refractive index. However, since the resin material has higher moisture-absorption characteristics than a glass material, the objective lens configured by such a resin material has a problem that spherical aberration is caused by the high moisture-absorption characteristics.